The present invention relates in general to intrusion alarm systems and more particularly to an intrusion alarm system that detects the presence of an intruder by sensing radiation energy.
Intrusion alarm systems have been known to sense variations in radiation energy to detect the presence of an intruder. The variations in radiation energy have been employed to activate an alarm. The optical device employed in such systems has been designed to reflect variations in radiation energy from a long, narrow beam that is highly directive, or from a wide angle, short beam. Wide angle systems have been manufactured by Raytek, a division of Optical Coating Laboratory of Santa Rosa, California. Passive infrared intruder alarms using Plessey pyroelectric detectors have been described in advertising literature published by Plessey Optoelectronics & Microwave Limited, Towcester, Northants, England. The article by Plessey Optoelectronics also describes pyroelectric detectors used in such systems. Advertising literature has also been published by Thermometrics, Inc., of Edison, New Jersey, on infrared detectors for intruder alarm systems, and particularly detectors employing thick film thermister flakes and segmented optical mirrors.
Heretofore, intruder alarm systems employed sensing mirrors with spherical sections. The radii of the spherical sections were the same. Thus, the radii were equal in the vertical direction and in the horizontal direction. Therefore, the surveillance of the target area comprised circular sections. As a consequence thereof, sensing mirror sections were placed below the main arrangement of sensing mirror sections to cover adequately the central section of the area under surveillance. Additionally, the sensing mirror sections were spaced apart.
Intruder alarm systems should be able to detect an intruder and also it is essential that the intruder alarm system be free from false alarms. In the wide angle, passive radiation intruder alarm systems, false alarms can be triggered by various events, such as temperature change in a confined space, vertically moving air currents, electrical noise, power line transients and the like.
Heretofore, false alarms in the passive, wide angle radiation intruder alarm systems were caused by air currents brought about by moving clouds, floor heaters, et cetera. Such air currents were in the nature of vertical air currents. False alarms in the passive, wide angle radiation intruder alarm systems were caused by radio frequency signals, such as those produced by microwave generating equipment, electric motors, RF oscillators and the like. When thermisters are employed as the sensing device, false alarms occur because the thermisters are sensitive to RF fields. In prior systems, the thermisters were disposed in front of the optical device at the most exposed location in the detection system because of the optical device employed, instead of adjacent the amplifier which would provide improved shielding from RF energy or the like.
In the patent to Berman, U.S. Pat. No. 3,703,718, issued on Nov. 21, 1972, for Infrared Intrusion Detection System, there is disclosed a passive, wide angle infrared intrusion detection system in which an optical device focuses infrared radiation for impinging on a sensing element. The optical device comprises a plurality of spaced apart, vertically disposed segments and has a concave reflecting surface. The segments are formed by cutting the mirror along a plurality of parallel planes, which are also parallel to the axis of the mirror. The reflective segments have substantially equal reflective areas. In a modification of the system, a sensing element is mounted in front of the segmented spherical mirror.
The patent to Sprout et al., U.S. Pat. No. 3,928,843, issued on Dec. 23, 1975, for Dual Channel Infrared Intrusion Alarm System discloses an intrusion alarm system utilizing two sensing elements and two signal processing channels such that an intruder produces signals of opposite polarities in the two channels. An alarm is triggered in the event two signals of opposite polarities are present at the same time. The optical device reflects infrared energy eminating from a plurality of spaced apart fields of view and focuses the energy onto a sensor. The optical device has a concave reflecting surface and includes five vertically extending segments with two horizontally extending segments disposed above the vertical segments. The sensors are associated with each of the vertical segments of the optical device.
In the patent to Cinzori et al., U.S. Pat. No. 3,955,184, issued on May 4, 1976, for Passive Infrared Room Intrusion Detector, there is disclosed an intrusion system in which one of the optical devices has concave reflecting surfaces. The reflective surfaces reflect orthogonal fields of view which are thin curtain-like polyhedrons coextensive with planar surfaces of the room. The patent to Falbel, U.S. Pat. No. 3,271,575, issued on Sept. 6, 1966, for Catoptric Radiometric Detector discloses an instrument for detecting optical radiation from a target in which the collecting optics are illustrated as a typical Cassegrain system with a primary mirror and a secondary mirror. A truncated cone is employed with a highly reflective inner surface and a sensor is mounted at the truncation. The patent to Rossin, U.S. Pat. No. 3,839,640, issued on Oct. 1, 1974, discloses a passive intrusion detection device in which a concave mirror has the sensor mounted on the focus of the front reflective surface of the concave mirror. In the patent to Stern, U.S. Pat. No. 3,708,222, there is disclosed a rearview mirror for a vehicle, which is a Fresnel mirror. The mirror is formed from a plurality of concentric annular prisms to provide a wider field of view.
Other patents of interest are:
Cruse--U.S. Pat. No. 3,524,180 PA1 French patent to Cupuano et al.--U.S. Pat. No. 1,464,783 PA1 Osborne--U.S. Pat. No. 3,480,775 PA1 Bradshaw et al.--U.S. Pat. No. 3,766,539 PA1 Schwartz--U.S. Pat. No. 3,631,434